The present invention relates to apparatus and methods for performing immunoassays. More particularly, the invention relates to apparatus and methods for performing immunoassays using surface acoustic waves on a sensor.
Generally, the term "antigen" is applied to any foreign substance which, when present in a host animal, will stimulate the production of specific antibodies. These antibodies show a remarkable ability to bind selectively the antigen which stimulated their production. This ability of the antibodies to discriminate between the antigens which stimulated their production and the numerous other compounds having a similar structure is the basis for virtually all immunoassay techniques. These techniques are based on the principal that an antibody will form a complex by selectively binding with the antigen which stimulated its production.
Numerous different immunoassays are known in the art for the qualitative and quantitative determination of antigens and antibodies. Immunoassay procedures such as radioimmunoassay and fluoroimmunoassay are the most widely used methods in immunochemistry. Despite their great utility in the analysis of materials of clinical and biomedical importance, these methods are limited by the hazards and short lifetimes of radioactive labels, expensive instrumentation, complicated procedures and relatively long incubation periods. Although sensitivities obtained by radioimmunoassays are high, alternative techniques are being developed to overcome the need for radioactive labels. These include enzyme linked immunosorbant assays (ELISA) and recent advances in fluoroimmunoassays as well as the use of ion-selective electrodes for clinical applications. However, these techniques suffer from the disadvantages of requiring chemical modification of the antigen, highly specialized antisera, very precise timing during the assay and reagent instability. Because of the problems associated with these methods of performing immunoassays, efforts have been made to develop new, simpler techniques and procedures.
One area which has shown some promise in providing a new procedure is the use of piezoelectric oscillators for detecting antigens and antibodies. Various types of methods and sensors have been developed. For example, U.S. Pat. No. 4,242,096 discloses an indirect method for determining an antigen in a liquid sample. In this procedure, a piezoelectric oscillator is coated with the antigen which is to be determined. The antigen-coated oscillator is then contacted with the liquid sample and a predetermined amount of an antibody specific for the antigen being determined. This antibody then reacts with the antigen on the sensor and the antigen in the test solution. After the reaction is completed, the sensor is removed and the change in its resonant frequency because of the antibody is determined. The amount of antigen in the solution can then be determined by comparing this change in frequency to a standard curve.
Another sensor and method is disclosed in U.S. Pat. Nos. 4,236,893 and 4,314,821. In this method, an antigen specific for the antibody to be determined is bound to the surface of a piezoelectric oscillator. The antigen-coated oscillator is then exposed to a solution containing an unknown amount of the antibody. After the antibody in the solution has attached to the antigen on the oscillator, the oscillator is exposed to a substance which selectively binds to a specific subclass of the antibody being determined. This substance is referred to as a sandwiching substance. The frequency of the oscillator is measured before and after exposure to the sandwiching substance. The change in frequency is related to the amount of the subclass of antibody bound to the oscillator and the amount of the subclass of antibody in the solution can be quantified by reference to a standard curve.
While these sensors have provided a significant advancement in the art for performing immunoassays, they still have certain disadvantages. First, these sensors utilize bulk mode acoustic waves which propagate through the entire crystal thickness from one surface to an opposing surface. Additionally, these sensors were designed such that they could only oscillate when a gas was present at the crystal surface and close control of temperature and humidity was maintained. Further, these sensors can generally only be utilized to perform a single immunoassay.
Accordingly, it would be a significant advancement in the art to provide a sensor and method for performing immunoassays in which the sensor could be utilized for repeated measurements. It would be a further advancement if the sensor could determine the amount of antigen or antibody present while still in solution, thus eliminating the need to dry the oscillator and carefully control the temperature and humidity. Such a sensor and method are disclosed and claimed herein.